The field of the present invention relates to string trimmer devices. A string trimmer is a rotary mowing device using a flexible filament as a cutting mechanism. A string trimmer typically uses a plastic filament line mounted on a spool and rotated through a drive means to create the cutting action. When breakage or wear of the line occurs through impact with objects, fatigue or the like, additional filament needs to be unwound from the spool to replace the lost portion. For efficient use of filament, it is generally desirable that only a given amount of line be unwound when required.
Early generation string trimmers had to be stopped to permit manual extension of line to replace the broken portion. Such a procedure was inefficient and undesirable. One approach for automatic feed is the "bump feed" mechanism such as disclosed in Proulx, U.S. Pat. No. 4,103,212 or as in Oberg, U.S. Pat. No. 4,524,515. The Oberg device includes a driven member with a spool insertable therein. Ratchet means are included on the driven member and the spool, and spacing is provided between the driven member and the spool to allow for axial displacement of the spool to engage and disengage the ratchet means. When a ground engaging hub is contacted with the ground, the spool is axially displaced disengaging the spool, allowing it to rotate thereby unwinding a length of line therefrom. The bump feed trimmer requires the spool and the ground contacting hub to be axially displaceable and the operator must "bump" the head of the trimmer hard enough to cause the axial displacement. Bumping the trimmer on the ground could damage the trimmer especially if the operator applies a greater "bump" than that needed to release additional line.
Other devices accomplish the line metering operation by stopping of the trimmer. One such device is disclosed in Foster, U.S. Pat. No. 4,426,780. The Foster device indexes out a length of line whenever the motor is disengaged (such as by turning it off) whereby the driven portion (which includes the spool of line) overruns the driving portion. The Foster device depends on a relation that the driving portion connected to the motor would experience greater drag than the driven portion upon stopping allowing the driving portion to be overrun. Such an overrunning device may have difficulties in accurately indexing out a certain length or require a complicated mechanism to control the indexing.
It is therefore desirable to have a string trimmer with automatic feed of alternate design, preferably one with a minimum of moving parts.